Internet Protocol (IP) multicast typically refers to one or more protocols for effectively sending data communications to multiple receivers at the same time on a TCP/IP network, utilizing a multicast address. Example protocols that support IP multicast include Protocol Independent Multicast (PIM), Distance Vector Multicast Routing Protocol (DVMRP), Multicast OSPF (MOSPF), Multicast BGP (MBGP), and the Internet Group Management Protocol (IGMP).
In an IP multicast network with routers between multicast sources and receiving hosts, multicast traffic may be transmitted through the network to a designated router (DR) of a receiving host. The designated router may be the last hop router in a path (e.g., specified in terms of a multicast distribution tree) to a receiving host. For example, a receiving host may execute IGMP v2 to inform the designated router of its interest in receiving a particular multicast group. Multicasting of television channels provides one example of an application that utilizes IP multicast technology. In this example, a television channel may be represented by a multicast group (e.g., a multicast group address G is used to identify the television channel (e.g. ABC channel 1) to the multicast network).
Continuing the above example, it is the responsibility of the designated router to read an IGMP v2 message (which contains the multicast group G) received from a receiving host, and to send a PIM message towards the source of the relevant multicast group (G). As the PIM message flows through the multicast network, a multicast path is built for the relevant multicast group. In this manner, a multicast tree is established between a source router and a designated router within the multicast network. When a source host sends a multicast stream, the stream accordingly flows down the tree to the designated router. With respect to the above mentioned PIM protocol, a particular implementation thereof is known as Source Specific Multicast (SSM). In the case of PIM-SSM, the above mentioned PIM message contains the IP address of both the source (S) and the group (G). The combination (S,G) is referred to as a “multicast channel”.
Of course, the source of a multicast group may fail, or network problems may cause a multicast stream to fail. The continued delivery of IP multicast traffic in such a situation presents a number of technical challenges.